Aluminum hydroxides, in general, and boehmite, in particular, are widely used in soft polishing formulations where the abrasive is mixed with either wax, grease, oil or water to form a block, paste or liquid suspension. The important properties of the abrasive which affect its polishing ability relate to the size, shape and friability (the property of readily crumbling under the pressure exerted during polishing) of the particles. In very general terms, coarse particles provide a cutting action but leave a poor surface finish, whereas fine particles will remove less material but can provide a good surface finish. Until now, a scratch on a plastic surface or a metallic finish would require both an abrasive and a buffing material. The present invention provides a process for the production of boehmite of a size and structure which enables the two polishing actions to be applied consecutively from a single abrasive powder, so that cutting action (to remove surface blemishes, scratches, etc.) and smooth surface finishing can be combined in a single polishing formulation.
It is known that boehmite is the thermodynamically stable phase of aluminum monohydrate at temperatures above 100.degree. C. in caustic aluminate liquors of the well-known Bayer process for the production of alumina from bauxite. Accordingly, fine crystals of boehmite can be precipitated out of these solutions at temperatures above 100.degree. C. Of greater commercial significance is the method of producing boehmite whereby aluminum trihydroxide (gibbsite) from the Bayer process is digested in water or steam under hydrothermal conditions (See, for example, U.S. Pat. No. 3,954,957 of Koenig issued May 4, 1976.) In the Gibbsite process, the presence of an acid or alkali is beneficial in accelerating the digestion and controlling the particle size of the boehmite product. Although the individual boehmite crystals produced by this process are generally very small, less than 1 micron, they can form aggregates of 40-60 microns in size. However, the aggregate particles are fragile and unstable, releasing many of the fine crystallites as free entities, which limits the usefulness of the thus prepared boehmite. While useful in a number of other applications, such prior art particles are not suitable for a polishing application where an initial cutting action followed by a surface finishing action is required. Both actions should be performed by a single formulation useful for polishing, for example, plastic spectacle lenses, synthetics, car paint, lacquered finishes and the like.
As described in copending U.S. patent application Ser. No. 847,634, now U.S. Pat. No. 4,668,486, a Bayer spent liquor, i.e., a liquor having a high Na.sub.2 O.sub.free /Al.sub.2 O.sub.3 molar ratio and a low alumina content, when subjected to organics oxidation (i.e., a process for removing from the Bayer liquors organic contaminants present in the original bauxite used) in the presence of copper catalyst ions, can be induced to precipitate about 50% of its alumina content as copper-containing boehmite. The boehmite recovered from this process has a relatively high amount of copper content between about 0.35% and 0.70%, depending upon the amount of copper used as a catalyst and the amount of boehmite crystallized. This process is conducted under hydrothermal conditions (elevated pressure and temperature) and the copper-containing boehmite is recycled as a catalyst/catalyst carrier system. No seeding of the liquor is necessary for boehmite crystallization. Moreover, the retention time requirement (the amount of time that the liquor to be oxidized must remain within the autoclave) for the combined organics oxidation - boehmite crystallization is of the order of one hour or less. However, the boehmite precipitated by this process is not suitable for soft polishing applications principally because of the small particle size of the resultant boehmite crystals.
The present inventor has unexpectedly discovered that if the organics oxidation of caustic aluminate liquors (i.e. the process of removing organic carbon contaminants from caustic aluminate, liquors, such as Bayer process liquors) is conducted under conditions of greatly reduced copper concentration, i.e. consistent with copper levels in boehmite up to no more than approximately 0.1%, then the particle size and the structure of the resulting boehmite aggregates are such as to render it well-suited for use in, for example, soft polishing applications, where both cutting and finishing actions are desireable in a single formulation.
It is an object of the present invention to provide a process for the production of large boehmite crystal aggregates of novel crystalline structure.
It is a further object of the present invention to provide a novel form of boehmite containing small amounts of copper suitable for use in soft polishing formulations.
These and other objects of the present invention will be apparent to those of ordinary skill in the art in light of the present description, accompanying claims, and appended drawings.